He touched down 35,756 feet below the ocean surface in the Challenger Deep, a depression within the Mariana Trench, itself a much larger trough at the intersection of two tectonic plates. In 2012, the film director James Cameron became the first person to dive solo all the way down the deepest gash on Earth. Figuring out why this planet has a movable crust could tell geologists more not just about this planet, but about all planets or moons with solid surfaces, and whether they could have life, too. Yet understanding how plate tectonics affects evolution - and whether it is a necessary ingredient in that process - hinges on finding answers to some of the hottest questions in geoscience: how and when the plates started moving. The phrase describes a planet with abundant water, an atmosphere and a landmass - all of which exchange and circulate material - as a prerequisite for life. In 2015, James Dohm and Shigenori Maruyama of the Tokyo Institute of Technology coined a new term for this interdependence: the Habitable Trinity. Some researchers even believe that without the movement of continents, life might not have evolved into complex forms. If plate tectonics did not force slabs of rock to dive underneath one another and back into the Earth, a process called subduction, then the seafloor would be entirely frigid and devoid of interesting chemistry, meaning life might never have taken hold in the first place. Without plate tectonics driving the creation of coastlines and the motion of the tides, the oceans might be barren, with life-giving nutrients relegated forever to the stygian depths. Earth’s oceans might not exist if water were not periodically subsumed by the planet’s mantle and then released. They have shown that Earth’s atmosphere owes its longevity, its components, and its incredibly stable Goldilocks-like temperature - not too hot, but not too cold - to the recycling of its crust. In the past few years, geologists and astrobiologists have increasingly tied plate tectonics to everything else that makes Earth unique.
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